JOHANNESBURG (AFP) - Two South African brothers arrested on terrorism charges were plotting to blow up the American embassy in Pretoria and Jewish institutions, and planned to join the so-called Islamic State in Iraq and Syria (ISIS), police said on Monday.

The 23-year-old twins, Brandon-Lee and Tony-Lee Thulsie, appeared briefly in a Johannesburg magistrate court after being arrested at the weekend.

"It is alleged that they wanted to bomb the US embassy and Jewish facilities in the country," Hangwani Mulaudzi, spokesman for South Africa's elite police, the Hawks, told AFP.

The United States and Britain warned last month of possible imminent "terrorist" attacks by Islamic extremists in South Africa's major cities.

The two men, who are accused of planning to join the ISIS group in Syria, will remain in custody and return to court on July 19 to allow for further investigation.

"They had been on our radar since 2015. They tried to leave the country twice, through OR Tambo airport and through Mozambique (to join ISIS)," Mulaudzi said.

According to court papers, the brothers "conspired to commit the crime of terrorism by planning to cause explosions at a mission of the United States of America and Jewish institutions."

The suspects were also accused of inciting other people to "aid and abet them" in the attack, according to the provisional charge sheet.

"The incitement was to further the political, religious or ideological motives and objectives of... the Islamic State in Iraq and the Levant (ISIL)," it said, using another name for ISIS.